(i) Field of the Invention
The present invention relates to a new process for producing a sialon and more particularly to a new process for producing a sialon by preparing in vapor phase a novel carbon-containing composition comprising silicon oxide, aluminum oxide and carbon and then calcining the carbon-containing composition in a nitrogen containing gas atmosphere.
(ii) Description of the Prior Art
Sialon is a compound represented by the formula (I) EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z (I)
having the crystal structure of Si.sub.3 N.sub.4 and it is a substituted solid solution with part of Si substituted by aluminum and part of N substituted by oxygen. A shaped ceramic product obtained by sintering a sialon powder is far superior in mechanical strength and corrosion resistance at high temperatures as compared with conventional metallic materials and is therefore expected to be used as the material of engine and gas turbine.
According to a typical conventional method of producing a sialon powder, as disclosed in U.S. Pat. No. 3,960,581, a clay which is a relatively intimate mixture of Al.sub.2 o.sub.3 and SiO.sub.2, and carbon powder, are mixed using a ball mill or the like and the resulting mixture is subjected to a reductive nitrogenation in a nitrogen containing gas atmosphere, e.g. N.sub.2. However, sintered or shaped ceramic products obtained from sialon powder by such conventional manufacturing method have been disadvantageous in that sintered or shaped product having constant mechanical strength can not be obtained. Because the sialon powder obtained from clay and carbon powder comprises coarse particles and therefore in order to obtain a fine powder required as a starting material of a sintered product it is necessary to pulverize the coarse sialon powder for an extremely long period of time by means of a pulverizer such as a ball mill or a vibration mill, but according to such a mechanical pulverizing method, even if the pulverization is performed for a considerably long period of time, it is actually difficult to obtain an intimately mixed, very fine sialon powder having a high mechanical strength and affording a ceramic sinter in good reproducibility. Besides, there is the problem that the operation process permits an easy incorporation of impurities.
Thus, the finer the sialon powder, the easier the sintering, and the higher the purity, the smaller the deviation in strength of the resulting sintered or shaped product. But in the case of a sialon powder obtained by the conventional method, it is impossible to ensure its fineness and purity high enough to satisfy such properties. Additionally, where a sialon powder is to be produced on an industrial scale, since the operation is performed according to a batch process, there are such problems as troublesome operations for mixing and charging the starting materials as well as an increase of cost due to a long-time pulverization, etc.